/*
 * Copyright 2012 The Netty Project
 *
 * The Netty Project licenses this file to you under the Apache License,
 * version 2.0 (the "License"); you may not use this file except in compliance
 * with the License. You may obtain a copy of the License at:
 *
 *   http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
 * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
 * License for the specific language governing permissions and limitations
 * under the License.
 */
package io.netty.util;

/**
 * An attribute which allows to store a value reference. It may be updated atomically and so is
 * thread-safe.
 *
 * @param <T> the type of the value it holds.
 */
public interface Attribute<T> {

  /**
   * Returns the key of this attribute.
   */
  AttributeKey<T> key();

  /**
   * Returns the current value, which may be {@code null}
   */
  T get();

  /**
   * Sets the value
   */
  void set(T value);

  /**
   * Atomically sets to the given value and returns the old value which may be {@code null} if non
   * was set before.
   */
  T getAndSet(T value);

  /**
   * Atomically sets to the given value if this {@link Attribute}'s value is {@code null}. If it was
   * not possible to set the value as it contains a value it will just return the current value.
   */
  T setIfAbsent(T value);

  /**
   * Removes this attribute from the {@link AttributeMap} and returns the old value. Subsequent
   * {@link #get()} calls will return {@code null}.
   *
   * If you only want to return the old value and clear the {@link Attribute} while still keep it in
   * the {@link AttributeMap} use {@link #getAndSet(Object)} with a value of {@code null}.
   *
   * <p>
   * Be aware that even if you call this method another thread that has obtained a reference to this
   * {@link Attribute} via {@link AttributeMap#attr(AttributeKey)} will still operate on the same
   * instance. That said if now another thread or even the same thread later will call {@link
   * AttributeMap#attr(AttributeKey)} again, a new {@link Attribute} instance is created and so is
   * not the same as the previous one that was removed. Because of this special caution should be
   * taken when you call {@link #remove()} or {@link #getAndRemove()}.
   *
   * @deprecated please consider using {@link #getAndSet(Object)} (with value of {@code null}).
   */
  @Deprecated
  T getAndRemove();

  /**
   * Atomically sets the value to the given updated value if the current value == the expected
   * value. If it the set was successful it returns {@code true} otherwise {@code false}.
   */
  boolean compareAndSet(T oldValue, T newValue);

  /**
   * Removes this attribute from the {@link AttributeMap}. Subsequent {@link #get()} calls will
   * return @{code null}.
   *
   * If you only want to remove the value and clear the {@link Attribute} while still keep it in
   * {@link AttributeMap} use {@link #set(Object)} with a value of {@code null}.
   *
   * <p>
   * Be aware that even if you call this method another thread that has obtained a reference to this
   * {@link Attribute} via {@link AttributeMap#attr(AttributeKey)} will still operate on the same
   * instance. That said if now another thread or even the same thread later will call {@link
   * AttributeMap#attr(AttributeKey)} again, a new {@link Attribute} instance is created and so is
   * not the same as the previous one that was removed. Because of this special caution should be
   * taken when you call {@link #remove()} or {@link #getAndRemove()}.
   *
   * @deprecated please consider using {@link #set(Object)} (with value of {@code null}).
   */
  @Deprecated
  void remove();
}
